This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Recent developments of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) aim to facilitate accessing local IP-based services in various places, such as at home, office, or public hot spots, or even in outdoor environments. One of the important use cases for the local IP access and local connectivity involves a so-called device-to-device (D2D) communication mode, wherein user equipments (UEs) in close proximity to each other perform a direct communication. This direct communication is typically over a distance of less than a few tens of meters, but may sometimes extend to hundreds of meters.
Because D2D UEs are much closer to each other than cellular UEs that have to communicate via at least one cellular access point (e.g., an eNB), the D2D communication enables a number of potential gains over the traditional cellular technique, including capacity gain, peak rate gain, and latency gain.
The capacity gain may be achieved, for example, by reusing radio resources (e.g., OFDM resource blocks) between D2D and cellular communications and by reducing the number of links between UEs from two to one and accordingly reducing the radio resources required for one link. The peak rate gain directly results from the relatively short distance between D2D UEs and the potentially favorable propagation condition there between. The latency gain is also a direct result of the single relatively short link between D2D UEs.
FIG. 1 illustrates an example of a mixed cellular and D2D network, wherein UE 101 is a cellular UE which communicates via an eNB 110, whereas UEs 102 and 103 are D2D UEs which communicate with each other directly. In such a mixed cellular and D2D network, D2D communications share radio resources with cellular uplink (UL) communications.
In the LTE context, it has been further proposed that the D2D data communications should use the same coding and modulation techniques as used by devices in communicating with the cellular network in uplink. The details about coding and modulation for LTE uplink communication are given by existing 3GPP standards, e.g., in the 3GPP documents “Multiplexing and Channel Coding”, 3GPP TS 36.212, v 10.2.0 (June 2011) and “Physical Channels and Modulation”, 3GPP TS 36.211, v 10.2.0 (June 2011), available at http://www.3gpp.org.
According to 3GPP TS 36.212, in the case of data coding, a rate-1/3 turbo encoder shall be used. The output of the turbo encoder consists of three streams, corresponding to one systematic bit and two parity bit streams (referred to as the “Systematic”, “Parity 1”, and “Parity 2” streams in the following), respectively, as well as 12 tail bits, due to trellis termination. Parity bits included in the two parity bit streams are collectively referred to as redundancy bits.
After being subjected to circular buffer rate matching (CBRM) and channel interleaving, the coded bits (including the systematic bits and the redundancy bits) go through processes, such as scrambling, modulation, layer mapping, precoding, Discrete-Fourier Transform (DFT) precoding, resource mapping and Single-Carrier Frequency-Division Multiple Access (SC-FDMA, sometimes called SC-OFDM) signal generation by Inverse Fast-Fourier-Transform (IFFT) operation, which are specified in 3GPP TS 36.211.
It can be shown that SC-OFDM is a time-domain modulation. That is, for a given SC-OFDM symbol, the first output time-domain samples contain energy mostly from the first “modulated symbols”, the last output time-domain samples contain energy mostly from the last “modulated symbols”, and the middle output time-domain samples contain energy mostly from the middle “modulated symbols”. Note the difference with OFDM where each modulated symbol occupies only one subcarrier, which in turn implies that each modulated symbol spans evenly all output time-domain samples.
Time-Division Duplexing (TDD) will be used as the duplex scheme of D2D communication, which means that the cellular uplink resources are allocated to up/down-stream directions of each D2D pair in a time-division multiplexing (TDM) manner. D2D devices (D2D UEs) are assumed to be half-duplex on the uplink resources during D2D communications.
On the resources used for these communications (called “UL resources” hereinafter), there are three kinds of subframes: cellular uplink subframes, D2D transmit (TX) subframes, and D2D receive (RX) subframes. These subframes can occur in several orders—thus, there will be different subframe transitions among these subframe transmissions, and there will be possible collisions at the beginning/end of subframes, in some cases causing interference to the beginning or ending portions of subframes. In many cases, these possible collisions are caused by the intentional differences in timing between UE transmissions and base station frame timing, known as timing advance (TA), and the corresponding timing advance needed for D2D communications, which may differ.
A few of these collision/interference scenarios are detailed below. It should be appreciated that there may be more. FIG. 2 illustrates an intra-UE interference scenario, e.g., where timing differences in a D2D transmission result in that a received D2D subframe at D2D UE2, labelled “D2D (rx),” collides with D2D UE2's attempt to transmit an uplink subframe to the cellular base station (eNB). As can be seen in the figure, the end portion of the received D2D subframe overlaps with the uplink cellular subframe at D2D UE2. Note that the part of the figure labeled “Only this part relevant for FDD” includes only uplink and D2D subframes. (The D2D subframes are marked “CS” at the eNB—this indicates that these are “combined subframes,” i.e., subframes that may contain D2D and/or cellular transmissions.) This part of the figure does not include any downlink or special subframes, which are present only if the eNB is operating in Time-Division Duplexing (TDD) operating mode. Thus it will be appreciated that the techniques described herein may be applied when the cellular system is operating in TDD mode or in Frequency-Division Duplexing (FDD) mode.
An assumption here is that the timing advance (TA) of D2D transmission uses the same TA as cellular. Because of different TA of D2D TX/D2D RX, propagation delay from D2D TX to RX and the ON/OFF transient period from D2D Rx to cellular Tx, there will be collision at the subframe transition from D2D RX to cellular UL.
FIG. 3 illustrates an inter-UE interference scenario, where reception of a D2D subframe (labeled “RX”) at D2D Rx is interfered by an uplink transmission by a nearby UE (“Cellular UE”). “C-SF” in this Figure indicates a combined subframe, which is a subframe that may contain D2D and/or cellular transmissions.
Again, because of different TA between the D2D TX and the cellular systems, and the propagation delay to D2D RX, the cellular UE will interfere with the D2D RX at the beginning of the D2D subframe. The figure shows a 2-microsecond interference, which corresponds to a 300-meter distance between the cellular UE and the D2D RX. If the distance is larger, such as 900 meters, corresponding to 6 microseconds of interference, then the interference cannot be handled by normal cyclic prefix and interference may also occur at the beginning of D2D subframe.
One proposed approach to address the possible interference discussed above is to modify the subframe used for D2D communications to include a guard period (GP) at the beginning and/or the end of the frame. For instance, to avoid the interference at the beginning and/or end of the D2D subframe, an OFDM symbol is reserved at the beginning and/or end of the D2D subframe. An LTE OFDM symbol corresponds to 70 us, which is large enough to avoid the inter-/intra-UE interference.
There are some remaining problems with this approach. For example, although it is a simple solution, a full blank symbol may cause an unnecessary resource waste. In practical D2D scenarios, the maximum distance between D2D TX and RX is only several kilometers, which correspond to several microseconds. The GP needed to accommodate potential collisions is on the same order. An OFDM symbol, on the other hand, is about 70 us. If an entire OFDM is reserved for the GP, it will cause great resource waste.